The films, and items produced therefrom, are particularly suitable for applications where flame protection or flame retardancy is demanded.
The film has a characteristic white appearance, making it particularly attractive is for the applications mentioned.
Sealable, biaxially oriented polyester films are known from the prior art. There are also known sealable, biaxially oriented polyester films which have been provided with one or more UV absorbers. These films known from the prior art either have good sealing performance, good optical properties, or acceptable processing performance.
GB-A 1465 973 describes a coextruded, two-layer polyester film in which one layer is composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters and the other layer is composed of polyethylene terephthalate. The specification gives no useful data concerning the sealing performance of the film. The film cannot be produced in a reliable process due to lack of pigmentation (the film cannot be wound) and it has restricted further processing capability.
EP-A 0 035 835 describes a coextruded sealable polyester film, the sealable layer of which has admixed particles to improve winding and processing performance, the average particle size exceeding the thickness of the sealable layer. The particulate additives form surface protrusions which inhibit undesired blocking and sticking to rolls or guides. No further information is provided on the incorporation of antiblocking agents with regard to the other, nonsealable layer of the film. It remains uncertain whether this layer comprises antiblocking agents. The use of particles having a larger diameter than the sealing layer and of the concentrations reported in the examples has an adverse effect on the sealing characteristics of the film. The specification provides no information on the sealing temperature range of the film. Seal seam strength is measured at 140° C. and found to be in the range from 63 to 120 N/m (0.97 to 1.8 N/15 mm of film width).
EP-A 0 432 886 describes a coextruded, multilayer polyester film which has a first surface on which has been arranged a sealable layer, and has a second surface on which has been arranged an acrylate layer. The sealable outer layer here may also be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters. The coating on the reverse side gives the film improved processing performance. The patent gives no indication of the sealing range of the film. The seal seam strength is measured at 140° C. For a sealable layer thickness of 11 μm the seal seam strength given is 761.5 N/m (11.4 N/15 mm). A disadvantage of the reverse-side acrylate coating is that this side is now not sealable with respect to the sealable outer layer, and the film therefore has only very restricted use.
EP-A 0 515 096 describes a coextruded, multilayer, sealable polyester film which comprises a further additive on the sealable layer. The additive may comprise inorganic particles, for example, and is preferably applied in an aqueous layer to the film during its production. Using this method, the film is claimed to retain its good sealing properties and to be easy to process. The reverse side comprises only very few particles, most of which pass into this layer via the recycled material. This patent again gives no indication of the sealing temperature range of the film. The seal seam strength is measured at 140° C. and is above 200 N/m (3 N/15 mm). For a sealable layer of 3 μm thickness the seal seam strength given is 275 N/m (4.125 N/15 mm).
WO 98/06575 describes a coextruded, multilayer polyester film which comprises a sealable outer layer and a nonsealable base layer. The base layer here may have been built up from one or more layers, and one of these layers is in contact with the sealable layer. The other (outward-facing) layer then forms the second nonsealable outer layer. Here, too, the sealable outer layer may be composed of isophthalic-acid-containing and terephthalic-acid-containing copolyesters, but these comprise no antiblocking particles. The film also comprises at least one UV absorber, which is added to the base layer in a weight ratio of from 0.1 to 10%. UV absorbers used here are preferably triazine, e.g. ®Tinuvin 1577 from Ciba Geiby (Basle, Switzerland). The base layer of this film has conventional antiblocking agents. The film has good sealability, but does not have the desired processing performance and has shortcomings in optical properties. The film may also have a matt surface, but then has high haze, which is undesirable.
DE-A 2346 787 describes a flame-retardant polymer. Besides the polymer, its use is also claimed for producing films and fibers. The following shortcomings were apparent when producing films with this claimed phospholane-modified polymer:                The polymer mentioned is susceptible to hydrolysis and has to be very thoroughly predried. When the polymer is dried using prior-art s it cakes, and it is therefore very difficult to produce a film.        The films produced under uneconomic conditions embrittle when exposed to heat, i.e. mechanical properties are severely impaired by embrittlement, making the film unusable. This embrittlement occurs after as little as 48 hours of exposure to heat.        
It was an object of the present invention to provide a white, sealable, flame-retardant and biaxially oriented polyester film which does not have the disadvantages of the prior-art films mentioned and in particular has very good sealability, can be produced cost-effectively, has improved processability, and has improved optical properties. It should particularly have flame-retardant action and not embrittle after exposure to heat.
It was an object of the present invention to extend the sealing range of the film to low temperatures, to increase the seal seam strength of the film, and at the same time to provide better handling of the film than is known from the prior art. It also has to be ensured that the processability of the film extends to high-speed processing machinery. It is intended that regrind arising directly during film production should be capable of reintroduction to the extrusion process at a concentration of up 60%, based on the total weight of the film, without any significant resultant adverse effect on the physical properties of the film.
Flame-retardant action means that in what is known as a fire protection test the film complies with the conditions of DIN 4102 Part 2 and in particular the conditions of DIN 4102 Part 1 and can be allocated to construction materials class B2 and in particular B1 for low-flammability materials.
It is also intended that the film pass the UL 94 “Vertical Burning Test for Flammability of Plastic Material” test so that it can be placed in class 94 VTM-0. This means that burning of the film has ceased 10 seconds after removal of the Bunsen burner, and that after 30 seconds no smoldering is observed, and also no burning drops arise.
Good mechanical properties include inter alia high modulus of elasticity (EMD>3200 N/mm2; ETD>3500 N/mm2) and also good values for tensile stress break (in MD>100 N/mm2; in TD>130 N/mm2).
Cost-effective production includes the capability of the raw materials or the raw material components needed to produce the flame-retardant film to be dried using commercially available industrial dryers, e.g. vacuum dryers, fluidized-bed dryers, or fixed-bed dryers (tower dryers). It is important that the raw materials do not cake and do not undergo thermal degradation.
No embrittlement on short-term exposure to heat means that after 100 hours of a heat-conditioning procedure at 100° C. in a circulating-air heating cabinet the film does not become brittle and does not have poor mechanical properties.